Catalytic dehydrogenation can be used to convert paraffins to the corresponding olefin, e.g., propane to propene, or butane to butene.
FIG. 1 shows one typical arrangement for a catalytic dehydrogenation process 5. The process 5 includes a reactor section 10, a catalyst regeneration section 15, and a product recovery section 20.
The reactor section 10 includes one or more reactors 25 (four are shown in FIG. 1). A hydrocarbon feed 30 is sent to a heat exchanger 35 where it exchanges heat with a reactor effluent 40 to raise the feed temperature. The feed 30 is sent to a preheater 45 where it is heated to the desired inlet temperature. The preheated feed 50 is sent from the preheater 45 to the first reactor 25. Because the dehydrogenation reaction is endothermic, the temperature of the effluent 55 from the first reactor 25 is less than the temperature of the preheated feed 50. The effluent 55 is sent to interstage heaters 60 to raise the temperature to the desired inlet temperature for the next reactor 25.
After the last reactor (in this example, the fourth reactor), the reactor effluent 40 is sent to the heat exchanger 35, and heat is exchanged with the feed 30. The reactor effluent 40 is then sent to the product recovery section 20.
The catalyst 65 moves through the series of reactors 25. When the catalyst 70 leaves the last reactor 25, it is sent to the catalyst regeneration section 15. The catalyst regeneration section 15 includes a reactor 75 where coke on the catalyst is burned off and the catalyst may go through a reconditioning step. A regenerated catalyst 80 is sent back to the first reactor 25.
As will be appreciated by those of ordinary skill in the art, the organic chloride used to condition paraffin dehydrogenation catalysts results in undesirable chlorinated species (chloride) compounds, such as HCl and organic chlorides (RCl), in the reactor effluent. Such compounds are referred to herein as trace chloride contaminants. Example deleterious effects from untreated trace chloride contaminants include corrosion, poisoning of downstream catalysts, and other effects. Accordingly, product recovery in typical catalytic dehydrogenation processes includes a process for removal of trace chloride contaminants.
For example, FIG. 1 shows a typical chloride removal process that is integrated into the product recovery section 20. The reactor effluent 40 is compressed in the compressor 82. The compressed effluent 115, which typically occurs at a temperature of about 110-177° C. (230-350° F.) when exiting the compressor 82 (e.g., via a discharge drum), is introduced to a cooler 120, for instance a heat exchanger. The cooler 120 lowers the temperature of the compressed effluent to about 25-60° C. and in some examples 38-60° C. (100-120° F.). The cooled effluent 125 (cooled product stream) is then introduced into a chloride remover 130, such as a chloride scavenging guard bed. The chloride remover 130 includes an adsorbent, which adsorbs chlorides from the cooled effluent 125 and provides a treated effluent 135. Treated effluent 135 is introduced to a drier 84. The drier 84 can be a reactor effluent drier system (RED) for drying and purification, including water and hydrogen sulfide (H2S) removal.
An example reactor effluent dryer (RED) system includes two or more adsorbent beds arranged in a typical thermal swing adsorption (TSA) system. While one or more adsorbent beds is in adsorption mode to purify and dehydrate the process stream, the other bed(s) are in regeneration mode. When the adsorbent bed(s) in the adsorption step starts to breakthrough the contaminants, the bed(s) on adsorbent mode is switched to regeneration mode and the freshly regenerated bed(s) are placed in adsorption mode. The beds are switched between adsorption and regeneration modes to provide for continuous purification of the process stream. Regeneration of the adsorbents is accomplished by purging the beds with a regenerant stream such as an inert gas, net gas, or vaporized hydrocarbon stream, at elevated temperature to desorb the impurities and water to rejuvenate the adsorbent and prepare it for a fresh adsorption step. The TSA process is well known to those skilled in the art.
The dried effluent is separated in separator 85. Gas 90 is expanded in expander 95 and separated into a recycle hydrogen stream 100 and a net separator gas stream 105. A liquid stream 110, which includes the olefin product and unconverted paraffin, is sent for further processing, where the desired olefin product is recovered and the unconverted paraffin is recycled to the dehydrogenation reactor 25.
An example chloride scavenging guard bed for the chloride treater 130 includes a vessel having one or more chloride scavenging adsorbents, referred to as guard beds. Examples of adsorbents include activated alumina, promoted aluminas, metal oxides, zeolite-based adsorbents, and others. The cooled compressed effluent 125 passes through the vessel, and over the beds, contacting the adsorbent to remove chloride contaminants. It is possible that both physical and chemical adsorption may take place.
A significant amount of heavy hydrocarbon residue that is present in the reactor effluent 40 enters the chloride treater 130. The heavy hydrocarbon residue is a result of undesirable side reactions occurring primarily in the reactor section 10 of the catalytic dehydrogenation unit. Examples of heavy hydrocarbons include polynuclear aromatics. The heavy hydrocarbon residue, a known impurity, negatively affects the performance of the chloride treater adsorbent. While a solvent, such as para-diethylbenzene or light cycle oil, can be provided to clean the reactor effluent 40 and the cooler 120, hydrocarbon residue still is present in feed to the chloride remover 130 or the reactor effluent driers 84.
If a reactive adsorbent is employed in a highly reactive hydrocarbon stream, such as that present in the catalytic dehydrogenation process, undesirable reactions, such as polymerization, alkylation, etc. can result in formation of high molecular weight heavier hydrocarbons. These heavy hydrocarbons can deposit on the adsorbent surfaces, filling up pore volume and creating mass transfer resistance by forming liquid film around adsorbent particles, thereby reducing capacity of the adsorbent. To compensate for such effects, it may be possible to increase the surface to volume ratio of the adsorbent particle by use of higher external surface area particles or reducing the particle size of the adsorbent. However, such strategies are not very effective, and may result in increased cost of production. In addition, reducing the adsorbent particle size will result in an undesirably large pressure drop across the chloride remover 130.